Circuits that produce stable reference voltages are necessary for establishing threshold voltages in data conversion devices, such as digital-to-analog or analogto-digital converters, or in digital logic circuitry. One reason why stable reference voltages are necessary is that variations in the offset and peak-to-peak voltages of switching signals applied to or developed by such circuits require a stable threshold reference to ensure that a voltage comparator will properly respond to transitions between voltage levels of the switching signal. Two popular logic families include emitter-coupled logic (ECL), which is implemented in silicon integrated circuit (IC) technology, and source-coupled logic, which is implemented in gallium arsenide (GaAs) IC technology. Voltage reference generator techniques suitable for one type of IC may not, however, be suitable for a different type of IC.
A resistive voltage divider and a series-connected diode circuit are two basic voltage reference circuit designs that are unacceptable for use as voltage reference generators in most integrated circuit applications. The resistive voltage divider is unacceptable because the output voltage changes in direct proportion to variations in the supply voltage applied to the voltage divider. The simple series-connected diode circuit is also unacceptable because the diode junction voltage is temperature sensitive, changing in accordance with the well-known temperature coefficient of approximately -1.5 mV/.degree.C. to -2.0 mV/.degree.C. for silicon diodes and of approximately -1.0 mV/.degree.C. to -1.5 mV/.degree.C. for GaAs Schottky diodes.
A bandgap reference circuit, such as that described in U.S. Pat. No. 4,714,872 of Traa, is implemented in a silicon IC and uses diodes and an operational amplifier to exploit the stability of the band gap to produce a constant output voltage. To effectively perform, a bandgap circuit requires a high gain, a low offset voltage amplifier, and a predictable base-to-emitter voltage of a bipolar junction transistor (BJT). A bandgap circuit is, however, unsuitable for use with gallium arsenide field effect transistor (GaAsFET) devices because they provide low gain and have a high voltage offset whose value can drift appreciably. (A typical GaAsFET amplifier provides a gain of about 10 and has a 50-100 mV offset that drifts at about 500 .mu.V/.degree.C. These parameters vary randomly for different GaAsFET devices.)
A voltage reference generator suitable for implementation in GaAsFET IC technology is described in U.S. Pat. No. 4,686,451 of Li et al. The Li et al. circuit uses two pair of Schottky diodes and a pair of depletion-mode metal semiconductor field effect transistor (MESFET) devices that are interconnected to produce a constant reference voltage. A drawback associated with the Li et al. circuit is that the nominal output voltage is sensitive to process parameters, thereby causing relatively large variations in output voltage of circuits produced in different process runs.